The present invention relates to communication devices such as telephones, and particularly cordless telephones.
The use of adaptive filtering technology to effect echo cancellation in conventional telephone devices is already known in the art. Conventional echo cancellation techniques are described, for example, by Messerschmitt, in a paper entitled Echo Cancellation in Speech and Data Transmission, IEE JOURNAL ON SELECTED AREAS IN COMMUNICATION, Vol. SAC-2, No. 2, March, 1984, 283-296.
Typically, a telephone set is connected by a single wire pair to a four-wire path forming part of the telephone utility. The four-wire path consists of two wires which conduct communication signals in one direction and two wires which conduct signals in the opposite direction. In order to prevent feedback in such a system, the two wires of the telephone set are connected to the four-wire path by a device known as a hybrid which is intended to provide signal isolation between the two wire pairs of the four-wire path.
A hybrid does not, however, prevent echos from being propagated in a loop between two connected telephone sets and when, in conventional telephone equipment, such an echo has a very long delay, as would be experienced in the case of connections via satellites, it is already known to use echo cancelers in the four-wire path near a telephone set. Such cancelers can be constituted, for example, by a finite impulse response (FIR) filter which, as is known, consists of a tapped delay line whose taps are connected to multipliers that multiply the signal at each tap by a filter coefficient, all of the multiplied signals being summed to produce a replica of the echo signal which is to be canceled.
It has further been found that echos will also occur in the hybrid itself due to inherent impedance mismatches associated with the hybrid. In a conventional telephone set, i.e. a telephone set in which the handset is connected to the telephone body by a wire, the echo delay through the hybrid is of such short duration that it is not noticeable to the user.
However, in the case of digital cordless telephones (DCTs) communication signals are transmitted between the handset and the base station in packets, each of which may contain about four milliseconds of audio data. Because of the resulting built-in delay in the transmission of audio data, an echo signal that passes through the hybrid will be perceivable to the user. This problem exists in the general class of Time Division Multiple Access (TDMA) systems.